Occipitocervical fixation systems provide stabilization of the base of the skull (or occiput) with respect to the neck. To function effectively, occipitocervical fixation systems should meet a number of design criteria. For example, such systems must be extremely strong and rigid, yet be minimally invasive and have a low profile. Such systems should also be relatively easy to assemble, implant, and maintain. Although spinal systems for the lumbar and the thoracic areas of the spine have been developed to meet these design criteria, the occipitocervical area presents additional challenges for implantation given its smaller size and increased concentration of nerves and arteries which need to be identified and avoided. Moreover, there is less muscle mass in the occiput to cover the implant.
Occipitocervical fixation systems typically include an occipital plate, which is secured to the occiput of the skull, rods on either side of the spine, which are fixed to vertebrae with screws or hooks, and rod receptacles, which attach the rods to the occipital plate. Of particular interest herein are the occipital plate and rod receptacles, referred to herein collectively as the occipital plate system. Recently, occipital plate systems have been developed in which the plate and rod components are discrete to permit greater flexibility during installation. Such occipital plate systems thus function to secure the rods to the occiput of the skull, while allowing for adjustment of the rods relative to the plate before the system is “fixed” usually by tightening the rod receptacles in some fashion.
For example, U.S. Pat. No. 6,477,790 discloses a plate and rod junction mechanism for securing the rod to the plate. The mechanism comprises a bolt with a slot to receive the rod, a rod support platform that fits over the bolt, and a nut, which tightens down on the bolt to urge the rod against the support platform and into the plate. The base of the bolt is round to rotate freely in a counter-bored aperture of the plate. The support platform is a thick, annular washer that lies over the base of the bolt and has a rounded groove to receive the rod. When installed, the plate is sandwiched between the base of the bolt and the support platform.
Although recently-developed systems, such as that disclosed in the '790 patent, provide flexibility/configurability, applicants recognize that they tend to have small “contact area” by virtue of the bolt residing in a slot. As used herein, the term “contact area” refers to the interfacial surfaces between the rod and plate, which are subject to compressive force when the occipital plate system is fixed in place. Generally, there must be adequate contact area between the rod and the plate—either directly or via inserts/washers—such that once the system is tightened, there is no relative movement between the rod and the plate. In the system of the '790 patent, the intersection of the plate and the rod is limited to just a portion of the edge of the aperture in which the bolt resides. To compensate for this limited contact area, the system of the '790 patent uses the support platform to essentially expand the contact area of the device to make it adequate to resist movement.
Although the support platform in the '790 patent increases the contact area between the rod and the occipital plate, its use faces a number of drawbacks. For example, it adds another component to the occipital plate system, which, in turn, increases the complexity of the system and complicates its installation. Additionally, because the support platform encircles bolt, it adds bulk to system, which can be problematic given the restricted space of the occiput.
Therefore, there is a need for an occipital plate system which tightly secures the rod to the plate, but which avoids bulk and complexity. The present invention fulfills this need among others.